Partner gone off sex?!?!

As soon as we found out we were expecting... he distanced himself from me in the bedroom, has made up all excuses to avoid sex :/ it can really get me down esp as I'm 18wks pregnant. My sex drive went at the beginning but soon returned lol he's even admitted that having sex with me whilst pregnant 'isn't right?' :( it makes me feel unwanted :S anyone else been in the same situation?